Cellular adhesion, migration and communication mechanisms have been recognized as crucial elements regulating the fate and function of leukocytes and their participation in immune responses. As such, the direct visalization of cell-cell and cell-pathogen interactions and the fine structural feature that influence these events is central to our understanding of how the immune system responds to antigen challenge. The Intravital Microscopy Core (Core B) has been formulated based on a stated need of PPG investigators and is designed to draw on the considerable experience of its key personnel to provide expertise and specialized equipment in the design and execution of in vivo imaging studies of immune responses to viral infections and other challenges. The Specifc Aims of the Intravital Microscopy Core are as follows: 1. To provide access and expertise in conducting specific experiments utilizing multi-photon microscopy recordings of intra- and extravascular blood cell adhesion and migration as well as cell-cell and cellpathogen interactions in as many as six dimensions (i.e. space, time, color, fluorescence intensity). 2. To offer a custom-designed environment with appropriate biosafety conditions for the generation, housing and intravital microscopy analysis of virally infected mice. 3. To execute and analyze intravital microscopy-based adhesion and migration studies in murine tissues. Currently available imaging models include: bladder; bone marrow; cremaster muscle; ear skin; knee joint; liver; popliteal and inguinal lymph node; small intestine, including mesentery, Peyer's patches and lamina propria; and spinal cord. 4. To provide assistance in the planning, execution and analysis of blood cell homing in vivo by providing standardized models of inflammation and infectious diseases and on-site fluorescent tagging of cells and viruses. 5. To provide computational resources for the storage, processing and in-depth analysis of digital and video-based imaging data. Services to be provided by Core B will allow participating investigators to investigate innate and adaptive immune cell behavior at a resolution ranging from single cell organelles to intact animals in a customdesigned environment dedicated to the study of replicating mammalian pathogens.